No worries - I certainly understand where you are coming from. The software is programmed such that plan changes initiated through the user interface will take place at the beginning of the next month however our support team can upgrade a user's plan for the current month when necessary. The primary situation in which we see this being necessary is if a user on the Starter plan decides to post a lot more listings than he/she previously thought such that the Standard plan would be less expensive.
We want to avoid allowing immediate plan changes, except for upgrades from the Free Tools plan, because allowing users to freely switch between plans would leave the system open to abuse. It would also create the possibility of needing to do an overcomplicated amount of proration with respect to billing.
You were right on point with respect to what the original poster experienced when he/she upgraded his plan using the user interface. I hope this clears things up
-Mike